Last year when we traveled to Europe we had to pay a tidy premium to our local bank to get a supply of Euros (we were arriving in Rome fairly late at night and our apartment had to be paid in cash). In speaking with a different banker this year he said he'd just charge the prevailing rate. And we live in a relatively small town. I won't pretend to know how this would be handled in the UK but even if you got large US bills, wouldn't the purser's office change that straight across? I understand that when they exchange currencies, it's different. But if you gave them a $50 and asked for 10s, 5s and 1s, I wouldn't think that would cost you anything extra. But I've no experience with that. And, honestly, now that alot of Europe is on Euros and pounds, it's not SUCH an inconvenience if the tippee(!) got them. Not like before when there were SO many different currencies. They go ashore also and need some amount of local currency. Good luck.